The entropy gathering process in OpenSSL varies between Linux and Windows due to differences in system calls and available entropy sources.
Linux
In Linux, OpenSSL primarily uses system calls to gather entropy. The most preferred method is to use the `getentropy()` or `getrandom()` system calls, depending on the Linux version and configuration[1]. These calls directly interact with the kernel to obtain entropy. If these calls are not available or fail, OpenSSL falls back to using file-based random number generators like `/dev/urandom`, `/dev/random`, and others[1]. Additionally, OpenSSL can use an entropy gathering daemon (EGD) as a last resort[1].
When generating random numbers using functions like `RAND_bytes`, OpenSSL adds additional entropy by incorporating system-specific data, such as the process ID (`getpid`) and time[2]. This ensures that even with the same initial seed, different random numbers are generated each time.
Windows
On Windows, OpenSSL does not have direct access to kernel-level entropy sources like Linux's `getentropy()` or `getrandom()`. Instead, it relies on the Windows Cryptographic API (CryptoAPI) or the Cryptographic Next Generation (CNG) API for entropy. The CNG API uses various entropy sources, including the Trusted Platform Module (TPM), CPU timing, and the RDRAND instruction if available.
When using OpenSSL on Windows, functions like `RAND_bytes` add entropy by incorporating system-specific data, such as the system time (`GetSystemTime`) and file time (`SystemTimeToFileTime`)[2]. This ensures that even with the same initial seed, different random numbers are generated each time.
Key Differences
- System Calls: Linux uses kernel-level system calls (`getentropy()` or `getrandom()`) for entropy, while Windows relies on higher-level APIs like CNG.
- Entropy Sources: Linux can use a variety of file-based entropy sources (`/dev/urandom`, `/dev/random`, etc.), whereas Windows relies on hardware and software entropy sources managed by the operating system.
- Additional Entropy: Both systems add additional entropy at runtime using system-specific data (e.g., process ID in Linux, system time in Windows).
Overall, both Linux and Windows ensure that OpenSSL's random number generation is robust and secure by incorporating diverse entropy sources and system-specific data.
Citations:
[1] https://lightshipsec.com/entropy-in-openssl-3-0/
[2] https://stackoverflow.com/questions/43869629/how-does-the-openssl-s-prng-works-in-windows
[3] https://hovav.net/ucsd/dist/debiankey.pdf
[4] https://crypto.stackexchange.com/questions/12571/good-entropy-source-for-generating-openssl-keys
[5] https://security.stackexchange.com/questions/47598/why-openssl-cant-use-dev-random-directly
What specific system calls are used by OpenSSL to gather entropy on Linux
On Linux, OpenSSL uses specific system calls to gather entropy, depending on the version of the Linux kernel and the configuration of the system. Here are the primary system calls used:
1. `getrandom()` System Call: This is the preferred method for obtaining entropy in modern Linux systems. Introduced in Linux kernel version 3.17, `getrandom()` allows applications to request random data directly from the kernel. OpenSSL uses this call to fetch entropy when available.
The `getrandom()` call can operate in two modes:
- Blocking Mode: If the system's entropy pool is depleted, the call will block until sufficient entropy is gathered.
- Non-Blocking Mode: If the system's entropy pool is depleted and the `GRND_NONBLOCK` flag is set, the call will return immediately with an error.
2. `getentropy()` System Call: Although not a standard Linux system call, some Linux distributions (like OpenBSD and possibly some Linux distributions that emulate OpenBSD's `getentropy()` for compatibility) might use this call. However, this is not typically used in standard Linux environments.
3. File-Based Entropy Sources: If the above system calls are not available or fail, OpenSSL falls back to using file-based entropy sources. The most common ones are:
- `/dev/urandom`: This is a non-blocking source of entropy that uses the kernel's entropy pool. It will return data even if the pool is depleted, using a pseudorandom number generator to fill the request.
- `/dev/random`: This is a blocking source of entropy that will only return data when the kernel's entropy pool is sufficiently filled. It is considered more secure than `/dev/urandom` but can block for extended periods if entropy is low.
OpenSSL uses these file-based sources by reading from them using standard file I/O operations (`open`, `read`, `close`).
4. Additional Entropy: Beyond these system calls, OpenSSL also gathers additional entropy by incorporating system-specific data, such as the process ID (`getpid`) and time (`gettimeofday` or similar). This ensures that even with the same initial seed, different random numbers are generated each time.
In summary, `getrandom()` is the primary system call used by OpenSSL on Linux for entropy gathering, with `/dev/urandom` and `/dev/random` serving as fallbacks.
What alternative methods does OpenSSL use to gather entropy on Windows
On Windows, OpenSSL uses alternative methods to gather entropy when the primary methods are not available or sufficient. Here are some of the alternative methods:
1. Windows Cryptographic API (CryptoAPI): OpenSSL can use the Windows CryptoAPI to gather entropy. This involves calling functions like `CryptGenRandom` to generate cryptographically secure random numbers. The CryptoAPI uses various entropy sources managed by the operating system.
2. Cryptographic Next Generation (CNG) API: This is a more modern API compared to CryptoAPI and provides better support for cryptographic operations, including entropy gathering. OpenSSL can use CNG functions like `BCryptGenRandom` to generate random numbers. The CNG API uses a variety of entropy sources, including hardware-based sources like the Trusted Platform Module (TPM) if available.
3. RDRAND Instruction: If the CPU supports it, OpenSSL can use the RDRAND instruction to generate random numbers. This instruction is part of the Intel Secure Key technology and provides a hardware-based source of entropy. However, this method is not typically used directly by OpenSSL but can be leveraged through the CNG API.
4. System Time and Other System-Specific Data: In addition to these APIs, OpenSSL also gathers entropy by incorporating system-specific data, such as the system time (`GetSystemTime` or `SystemTimeToFileTime`) and other environmental factors. This ensures that even with the same initial seed, different random numbers are generated each time.
5. Entropy Gathering Daemons (EGD): Although less common on Windows, OpenSSL can also use an EGD if configured to do so. An EGD is a separate process that continuously gathers entropy and provides it to applications that need it.
Key Considerations
- API Availability: The choice of API depends on the version of Windows and the availability of specific cryptographic APIs.
- Hardware Support: The use of hardware-based entropy sources like TPM or RDRAND depends on the hardware capabilities of the system.
- Configuration: OpenSSL's configuration can influence which methods are used and in what order.
Overall, OpenSSL on Windows relies on a combination of system APIs and hardware capabilities to ensure robust entropy gathering for secure random number generation.
Are there any third-party libraries OpenSSL uses for entropy gathering on Windows
OpenSSL itself does not typically use third-party libraries for entropy gathering on Windows. Instead, it relies on the built-in Windows APIs like the Cryptographic API (CryptoAPI) and the Cryptographic Next Generation (CNG) API for entropy. However, developers might choose to integrate third-party libraries with OpenSSL to enhance or customize entropy gathering for specific needs.
Here are some scenarios where third-party libraries might be used:
1. Entropy Gathering Daemons (EGD): Although not commonly used on Windows, an EGD can be implemented as a third-party solution. An EGD continuously gathers entropy and provides it to applications, including OpenSSL, if configured to do so.
2. Hardware-Based Entropy Sources: Some third-party libraries might provide access to specialized hardware-based entropy sources, such as USB-based random number generators or other external devices. These libraries would interact with the hardware and provide the gathered entropy to OpenSSL.
3. Custom Entropy Sources: Developers might create or use third-party libraries that gather entropy from custom sources, such as network packets, disk access times, or other environmental factors. These libraries would need to be integrated with OpenSSL manually.
4. Cross-Platform Entropy Libraries: There are libraries like `libentropy` or similar projects that aim to provide a cross-platform way to gather entropy. These libraries might be used in conjunction with OpenSSL to ensure consistent entropy gathering across different operating systems.
Key Considerations
- Integration Effort: Using third-party libraries requires additional integration effort, as they are not part of the standard OpenSSL distribution.
- Security and Reliability: The security and reliability of third-party libraries must be thoroughly evaluated before use in production environments.
- Compatibility: Compatibility with different versions of Windows and OpenSSL must be ensured.